1. Field of the Invention
The present invention relates to an apparatus and method for setting a desired thermodynamic state and for facilitating the attainment of thermodynamic equilibrium and measurement of interfacial tension and phase behaviour. The invention is most applicable when a phase separation occurs at the specific thermodynamic state between a mixture of components having different compositions and densities. In addition, this invention relates to an apparatus and method for determining engineering properties of the mixture to facilitate extraction and refining of relevant components of fluid mixtures. These engineering properties include interfacial tension between the phases, and volumetric behaviour during isothermal or constant composition expansion and differential liberation.
2. Description of the Prior Art
It is well known in the field of reservoir engineering and in the chemical engineering technology referred to as supercritical extraction, that extraction of oil/gas/water mixtures from a host reservoir depends on the physical properties of these phases and the interfacial tension between them, which vary from situation to situation. In order to improve efficiency in extracting oil, well-known tests have been developed to determine the various physical characteristics of the phases in a particular situation. One of the principal elements of interest is the interfacial tension between phases of differing density that occurs during extraction processes. One method of determining interfacial tension is by measuring the shape properties of drop of a dense phase suspended in a less dense phase (the gas). This is generally referred to as the "pendent drop" technique. An example of a device used to measure interfacial tension using the pendent drop method is the pendent drop interfacial tension cell sold by Temco, Inc. of Tulsa, Okla. Manipulation of interfacial tension between two or more phases during extraction can be achieved by changing pressure or composition of the phases allowing engineers to improve the rate of extractant recovery during the extraction process.
Another important parameter associated with the phase behaviour of oil and gas mixtures or oil/gas/water mixtures relates to the relative volumes of the more dense and less dense phases, taken at several pressure/volume/temperature points of the mixture.
Often experiments involve isothermal expansion of constant mass mixtures, sometimes referred to as "constant composition expansion" or "fluid compressibility," are carried out. The sample is initially a homogeneous single phase, the pressure of the sample being above its saturation pressure. Total volume of the system is increased, with a corresponding pressure decline until a lower pressure limit is reached, determined by a phase transition attained by determining a discontinuity in the compressibility or by the occurrence of minute droplets (bubble point) of the mixture.
A further experiment is the differential liberation measurement which requires stagewise separation of the gas portion of a mixture by segregating the oil and gas phases as volume is increased and pressure declines. At each step the total gas is removed and volume of the gas phase, liquid phase and total volume are determined. This procedure is important in estimating the total volume of oil and gas in the reservoir available for production.
All of the above tests and measurements require that the mixture be at, or close to, thermodynamic equilibrium in order to obtain reliable results. In addition, some tests require separation of the oil and the gas phases to obtain, for example, volume measurements (as with differential liberation).
In the past, samples had to be removed from the reservoir or process site to a sophisticated testing laboratory to perform these various measurements and tests. In petroleum reservoir engineering applications, thermodynamic equilibrium of the mixture was attained by inserting the oil/gas sample in a fixed volume cell containing liquid mercury. The cell would then be vigorously shaken and the mercury in the cell would physically aid the oil and gas mixture to attain thermodynamic equilibrium. Measurements and tests would then be undertaken on the oil and gas sample. However, mercury is a relatively toxic and dangerous substance to be handled, particularly in a non-laboratory setting. At the same time, to facilitate optimal extraction of oil and gas mixtures, it is important to have test results and measurements provided to them at the reservoir site quickly. The use of a cell containing the mercury is also an inappropriate method of attaining thermodynamic equilibrium in certain situations because of chemical reactions between the mercury and the sample.
Attempts have been made to develop a mercury-free apparatus which would enable rapid attainment of sample thermodynamic equilibrium. One such apparatus has been developed by Ruska Instrument Corporation of Houston, Tex. as Model 2370PVT System. In order to reduce the time needed to reach thermodynamic equilibrium, the sample mixture is mixed through the use of an internal mixing ring, which is magnetically coupled to an external mixing collar, which physically moves the mixing ring within the cell. Our apparatus offers mercury-free advantages while also making it possible to measure interfacial tension and other mixing features.